1. Field of the Invention
The present invention relates to an optical branching-coupling device usable as an optical branching device and an optical coupling device, and an optical module containing the same, and a manufacturing method of an optical branching-coupling device.
2. Description of the Related Art
In optical communication, optical branching-coupling devices and optical modules containing an optical branching-coupling device and a light emitting device and a light receiving device have a crucial role. For example, in single wire bidirectional optical communication, an optical branching and coupling device which branches and couples an optical waveguide for propagating the light emitted from a light emitting device such as an LED, an LD, or the like and an optical waveguide for propagating the transmitted light to a light receiving device such as PD is required at input-output terminal of the system. Meanwhile, the applicants of the present application have developed a number of self-written optical waveguides having a long axial core which is formed by using s self focusing caused in a curing resin when light-curing resin liquid is irradiated with light from an optical fiber or the like. And the applicants filed patents on such waveguides, some of which are disclosed in Japanese patent No. 4011283 and Japanese Unexamined Patent Application Publication Nos. 2002-365459, 2004-149579, and 2005-347441. At that time, the applicants also proposed that as an optical branching-coupling device or an optical multiplexer-demultiplexer device used in a single wire bidirectional optical communication, a branched core be formed of the above-mentioned self-written optical waveguide which is branched by a half mirror or a wavelength selective mirror. This suggests that an optical module having a light emitting device and a light receiving device may be easily formed.
An example of the structure of an optical module employing a self-written optical waveguide will be simply described below. FIG. 6 is a schematic diagram of an optical module 900 employing a self-written optical waveguide. The optical module 900 has a transparent housing 90, a photodiode (PD) 40, a light emitting diode (LED) 50 and a receptacle 902, and an external housing 91 covering these elements. The receptacle 902 is connected with a plastic optical fiber (POF) 20 through a connector 201. The transparent housing 90 has a half mirror 60 and three optical waveguide cores 39, 34, and 35 which form a branch by using a half mirror 60. The optical waveguide core 39 is optically connected with the receptacle 902 and the half mirror 60. The optical waveguide core 34 is optically connected with the PD 40 and the half mirror 60. The optical waveguide core 35 is optically connected with the LED 50 and the half mirror 60. In this structure of the optical module 900, an optical signal from an external optical line is input through the POF 20, passes through the connector 201, the receptacle 902, and the optical waveguide core 39 in that order; and is reflected by the half mirror 60, passes through the passing optical waveguide core 34; and then is introduced to the PD 40. Meanwhile, an optical signal from the LED 50, passes through the optical waveguide core 35, is transmitted by the half mirror 60; passes through the optical waveguide core 39, the receptacle 902, and the connector 201 in that order, and is output to the external optical line through the POF 20.
When the branched core is formed of the above-mentioned self-written optical waveguide in which a half mirror is utilized, optical branching or optical coupling is not always ideally conducted in the branching part. For example, in the optical module 900 of FIG. 6, the half mirror 60 with a reflectance of 50% and a transmittance of 50% used for arbitrary wavelengths invariably causes a loss of 3 dB of light received from the external optical line. After the addition of the loss occurring in the optical waveguide, specifically at a branching part due to other causes, to the loss occurring at the half mirror, the insertion loss of the optical branching-coupling device becomes about 6 dB.
The half mirror is typically produced by laminating a dielectric multilayer on a transparent substrate of glass or the like. Thus, the half mirror is expensive, causing the production cost of the optical branching-coupling device to be increased. In addition, the half mirror typically employs a substrate and a dielectric multilayer of inorganic material, which has weak adhesion to a light-curing resin which is an organic compound. For example, adding a thermal history to the optical branching-coupling device during a reliability test often causes detachment of the half mirror. This means that the detachment occurs even in practical usage with the lapse of time.
As described above, according to the related art, an optical branching-coupling device employs a half mirror, which has problems of the high insertion loss, high cost, and easy detachment of the half mirror from the optical waveguide. Then, the inventors of the present invention have deliberated on the production of an optical branching-coupling device in which no half mirrors are used, and completed the present invention.